HIV-1 infection of the central nervous system (CNS) occurs in a majority of AIDS patients and causes a variety of neurologic dysfunction and neuropathologies generally termed neuroAIDS. Microglia/macrophages, and astrocytes to a less extent are the main target cells for HIV-1 infection in the CNS, whereas neurons are rarely infected by HIV-1 but mostly affected in HIV/neuroAIDS. Therefore, several indirect mechanisms have been proposed for HIV/neuroAIDS pathogenesis. Among them is HIV-1 Tat protein. We have shown that Tat expression in the absence of HIV-1 infection is sufficient to cause neurobehavioral abnormalities and pathologies similar to most of those noted in HIV/neuroAIDS. Moreover, we have shown that Tat activates glial fibrillary acidic protein (GFAP) expression in astrocytes and results in astrocyte dysfunction and subsequent neuron death. Furthermore, our preliminary studies have found that Tat-activated GFAP expression involves a network of transcription factors and is associated with GFAP aggregates and endoplasmic reticulum (ER) stress in astrocytes and impaired neuron survival. Importantly, we have also obtained preliminary evidence to link the cerebrospinal fluid (CSF) levels to HIV/neuroAIDS pathogenesis. As a logical extension of our studies, we propose to further dissect the GFAP function in Tat neurotoxicity and HIV/neuroAIDS pathogenesis. Besides, we will determine the feasibility of using the CSF GFAP level as a novel HIV/neuroAIDS biomarker. Thus, the underlying hypothesis for the current proposal is that Tat adversely affects astrocyte function and neuronal survival through GFAP activation/aggregation and ER stress. In other words, GFAP is not only a mediator but also an indicator of Tat neurotoxicity and HIV/neuroAIDS pathogenesis. To test this hypothesis, we propose to address the following interrelated specific aims: (1) To characterize the relationship between GFAP expression and ER stress in astrocytes; (2) To determine effects of Tat-activated GFAP expression/aggregation and ER stress on astrocytes; (3) To define the molecular mechanisms of GFAP-/ER stress-mediated neurotoxicity; and (4) To investigate the potential of using GFAP as a novel HIV/neuroAIDS biomarker. We will use a combined molecular, cellular, biochemical, and genetic approach, including use of primary mouse cortical astrocyte cultures and neuron cultures, Tat transgenic mice, GFAP-null/Tat transgenic mice, primary human fetal brain cultures, embedded brain tissues and CSF samples of a large HIV-1 cohort in our studies. The answers sought have fundamental significance for understanding of this critical and pervasive protein GFAP, and its role in HIV/neuroAIDS pathogenesis. In addition, these answers shall also aid in identification of HIV/neuroAIDS biomarkers and development of anti-HIV/neuroAIDS therapeutic strategies.